1. Field of the Invention
This invention relates to a method and system for increasing the output power of gas lasers and more particularly to increasing the output power of sealed-off, diffusion cooled, CO2 waveguide lasers utilizing radio frequency (RF) excitation
2. Prior Art
The output power per unit cross sectional area for diffusion cooled CO2 lasers scales inversely as the square of the diameter of the discharge region and directly with the product of the mean free path and the thermo molecular speed of the CO2 molecules within the discharge region. CO2 diffusion cooled lasers have the advantages of smaller, size, longer sealed-off life time, and lower maintenance requirements below 500 to 1000 W of output power; while convectively cooled lasers, utilizing the fast flowing of the gas through the discharge region, have the advantage of higher power output capability ranging up to several tens of thousands of watts. When the product of the discharge diameter and the gas flow velocity is smaller than the product of the mean free path and the thermo molecular speed of the CO2 molecule within the discharge, higher power output per cross sectional area of the discharge is obtained with diffusion cooled lasers than with convectively cooled laser. (Review of CW High Power CO2 Laser, by Anthony J. DeMaria, Proceeding of the IEEE, pages 731 to 748 June 1973, which is incorporated herein by reference).
It is well know that diffusion cooled lasers utilize the collision of gas molecules, which have given up photons into the laser feedback cavity by stimulated emission but have not been completely de-excited to the ground state, with the walls of the housing containing the discharge to cool the gas within the discharge by de-exciting them to the ground state. This is especially true with CO2 molecules in typical CO2:N2:He discharges used in CO2 lasers. These wall collisions de-excite these CO2 molecules that have contributed a photon to the laser process down to the ground state, thereby cooling the discharge. The discharge containing housing is in turn cooled externally by either liquid or air cooling techniques. Air-cooling is utilized for lower power lasers that typically operate below 50 Watts of output power. It is known that if the cross section of the gas discharge section is large, the time required for CO2 molecules, e.g., in the center of the discharge, to diffuse to the cooled walls and became de-excited to where they can again participate in the stimulated emission laser process, is long. Consequently, the gas-cooling rate will be lower for diffusion cooled lasers that utilize large diameter discharges than for CO2 diffusion cooled laser whose discharges have smaller cross section. This results in lower power per laser beam cross sectional area as the cross sectional area of the discharge CO2 diffusion cooled laser is increased. The power output for diffusion cooled circular discharges scales as the inverse of the discharge tube diameter. As a result, the output power of diffusion cooled lasers with circular or square discharges can, to first order, only be increased by increasing the length of the discharge (Compact Distributed Inductance RF Excited Waveguide Gas Lasers by Leon A. Newman, John T. Kennedy, Richard A. Hart, U.S. Pat. No. 4,787,090, Nov. 22, 1988; Extended Multiple Folded Optical Path Laser, by Armando Cantoni, U.S. Pat. No. 5,610,936 issued Mar. 11, 1997, which are incorporated herein by reference).
Increasing the discharge length of diffusion cooled lasers beyond a convenient and practical length is usually accomplished by folding the discharge into some form of a closely packed zigzag pattern to obtain small, compact, rugged, and rigid laser head packages (Recent Research and Development Advances in Sealed-Off CO2 Lasers, by Leon A. Newman and Richard A. Hart, Laser Focus/Electro-Optics, June 1987, which is incorporated herein by reference). Utilizing the concept of U.S. Pat. No. 5,610,936, Armando Cantoni extended this concept of multiple folded optical path square waveguide shaped laser configuration to an unfolded single mode waveguide length of approximately six meters. With this six meter length, approximately 200 watts of output power was obtained with approximately thirtyfive optical bounces off multiple folding mirrors. Unfortunately, the impedance difference seen by the solid state RF source driving the large area discharge before the discharge is ignited compared to after it is ignited is so large that lighting the discharge and maintaining the discharge with one phase matching network structure is difficult. Distributed induction for tuning out the capacitance is used in Tuned Circuit RF Excited Laser, by Peter P. Chenausky, Errol H. Drinkwater, Lanny M. Laughman, U.S. Pat. No. 4,363,126 issued Dec. 7, 1982, which is incorporated herein by reference. U.S. Pat. No. 4,787,090 utilized spiral distributed inductors to achieve the tuning out of the capacitance taught by U.S. Pat. No. 4,363,126.
The output power of diffusion cooled lasers can also be increased by utilizing a rectangular discharge containing section. The two closely spaced walls of the rectangular discharge configuration provides good diffusion cooling while the other two walls of the rectangular discharge housing that are located far apart providing an increase in gas volume. This increase is gas volume yields higher output powers for a given length of laser. These rectangular discharge lasers are called slab lasers (Power Scaling of Large Area Transverse RF Discharge CO2 Lasers, by K. M. Abranski, A. D. Colley, etc., Applied Physics Letters, Volume 54 page 1833, 1989, which is incorporated herein by reference). CO2 slab laser technology has been responsible for pushing the average power output of diffusion cooled lasers to approximately the 1000 W range. Slab lasers normally yield multimode, large divergent beams unless the use of more complex optical feed back resonators, such as unstable resonators, are utilized to discriminate against the higher order modes.
Referring now to prior art FIGS. 1A-1D, the general types of RF excited diffusion cooled laser discharge configurations known today and normally found in presently commercially available CO2 laser heads with the exception of FIG. 1D are illustrated.
FIG. 1A illustrates the cylindrical discharge configuration, which usually utilizes either a glass or ceramic tube 2a. This configuration was the first to be utilized in diffusion cooled lasers dating back to 1972 for RF excited discharges and dating back to the mid 1960""s for DC excited discharges. In general, RF excitation has advantages over DC excitation predominantly because (i) the electrodes 4a, 6a are external to the discharge region 10, (ii) low voltages are utilized, and (iii) RF excitation is more compatible with solid state electronics. For the RF excited discharge arrangements, electrodes 4a, 6a are placed opposite one another down the outside length of the tube 2a across which an RF voltage is applied to excite the discharge. Larger diameters result in multiple modes unless more complex optical resonators are used, while smaller diameters (about several millimeters or less) result in waveguideing action that yield single mode beams with simple optical resonators configuration. CO2 diffusion cooled laser operation in a BeO capillary was reported in 1972 (BeO Capillary CO2 lasers by E. G. Burkhardt, T. J. Bridges, and P. W. Smith, Optical Communication, Volume 6 pages 193-1951, October 1972, which is incorporated herein by reference). Larger diameter tubes result in lower output power per unit discharge cross-sectional area because of the 1/D2 power output scaling characteristics mentioned previously. D is the tube diameter. For waveguide lasers, flat mirrors in closed proximity to the ends of the waveguide are normally used which greatly simplify the optical resonator.
The ground electrode 4a, 4b, 4c, 4d is normally part of the metal vacuum tight housing for all the configurations shown in FIGS. 1A-1D. This housing arrangement provides for good electromagnetic interference shielding and for good thermal conduction for either air or liquid cooling of the housing 8a, 8b, 8c, 8d. 
FIG. 1B illustrates the square discharge configuration 10b where the height (H) and width (W) are equal. This is the second oldest diffusion cooled waveguide CO2 laser technology. It dates back to the late 1970""s. This technology was utilized mainly for military laser radar and infrared (IR) counter measures applications in the past. This is the first configuration where waveguide folding was used to increase the length of the discharge to scale up the power for a given laser head length (Recent R and D Advances in Sealed Off CO2 Lasers by Leon A. Newman and Richard A. Hart, Laser Focus/Electro-Optics; June, 1987, which is incorporated herein by reference). In the mid 1990""s this technology began to be available for industrial applications. The same general comments regarding diffusion cooling and dimension scaling that were made for FIG. 1A apply to FIG. 1B. The insulator 12b separating the hot electrode 6b and the ground electrode 4b which is part of the laser housing 8b) is normally a low cost ceramic, which serves both as a good electrical insulator and a thermal conductor with excellent vacuum compatibility. Either air or liquid cooling can be used to conduct heat away at the location of the ground electrode 4b. Additional cooling can be obtained from the sides of the laser discharge housing 8b if required because of the good thermal conductivity of the ceramic and the metal housing 8b. Note that all of these basic discharge configurations provide space for a gas ballast 14b region for the CO2:N2:He gas mixture. This gas ballast contributes to the long sealed off lifetime of diffusion cooled lasers. Gas pressure in all the illustrated configurations of FIGS. 1A-1D normally range from several tens of torr to several hundreds of torr depending on the RF drive frequency and operating characteristics desired. Higher RF frequencies enable operation at high pressures which result in higher output power for a given discharge volume at the expense of higher cost associated with the power transistors.
U.S. Pat. Nos. 4,787,090 and 5,610,936 and patent application Ser. No. PCT/US98/05055, RF Excited Waveguide Laser, by R. A. Hart, J. T. Kennedy, E. H. Mueller and and L. A. Newman; filed on Mar. 13, 1998 based on U.S. Provisional Patent Application No. 60/041,092 filed on Mar. 14, 1997, which are incorporated herein by reference, discuss several approaches to waveguide folding to obtain a long discharge laser gain region for scaling to higher power for diffusion cooled waveguide gas lasers. If the electrodes 4b, 4c, 4d, 6b, 6c, 6d of FIGS. 1B through 1D are separated from the discharge region such as by the dielectric tube 2 in FIG. 1A, the output of the laser is not polarized. If one or both of the metal electrodes 4b, 4c, 4d, 6b, 6c, 6d are directly exposed to the laser radiation in the waveguide as in FIG. 1B through 1D then the output laser radiation is polarized parallel to the surfaces of the electrode(s).
Referring to prior art FIGS. 2A and 2B, two folded waveguide versions 16a, 16b for use in the configuration of FIG. 1B, utilized in commercially available waveguide lasers at the present time as discussed in U.S. Pat. No. 4,787,090 and patent application Ser. No. PCT/US98/05055 are generally shown. U.S. Pat. No. 5,610,936 describes a more elaborate folding arrangement and describes a rectangular ceramic folded diffusion cooled CO2 waveguide structure that contains two triangular end sections in which a grid waveguide structure consisting of two sets of parallel waveguide channels intersecting at right angles and optically coupled by the strip mirrors placed along edges of the triangular end sections. This approach has yielded approximately 200 W of output power. The xe2x80x9cNxe2x80x9d folded waveguide 18 of prior art FIG. 2A has yielded approximately 75 watts of output power in a laser head having dimensions of approximately 24 inches (L)xc3x973 inches (W)xc3x972.6 inches (H) and a total unfolded waveguide length of approximately 1.4 meters. The folded waveguide 20 of prior art FIG. 2B is folded in the shape of an xe2x80x9cNV.xe2x80x9d It typically yields 145 W of output power in a laser head having dimensions of approximately 24 inches (L)xc3x974 inches (W)xc3x973 inches (H) and a total unfolded waveguide length of approximately 2.25 meters. Comparing these results reveals that the waveguide configurations of FIGS. 2A and 2B are preferred over the configuration disclosed in U.S. Pat. No. 5,610,936.
There is one disadvantage in having a CO2 laser cavity that is too short. This has to do with the narrow line width of the CO2 molecules at the pressures of interest for use in CO2 lasers. The CO2 laser line broadens with pressure at approximately 5 MHz per torr. At 100 torr, the CO2 homogeneously broadened laser line is only approximately 500 MHz wide. Consequently, the length of the CO2 laser feedback cavity has a large effect on the output power stability as a function of temperature. This occurs because the optical frequency separation between adjacent axial modes of the lasers optical cavity is given by the velocity of light divided by twice the length of the cavity. Consequently for a cavity length of 20 cm, the optical frequency separation of the axial modes is 750 MHz while for a 100 cm long cavity, the axial modes are separated by 150 MHz. Consequently, the 100 cm long cavity laser has approximately five times more axial density when compared with the 20 cm long laser cavity. As the temperature of the laser varies with time, the frequency of these axial modes move through the laser gain bandwidth region. As one axial mode moves past the peak of the laser gain curve, output power begins to decrease. The output power will continue to decrease until the next adjacent axial modes have achieved higher gain and it begins to oscillate. This oscillation turn on of one axial mode and the oscillation turn off of a previously oscillating axial mode as they move across the peak of the gain curve causes the output power to vary as the temperature of the laser (i.e. the optical resonator) varies with time. The actual output power variation is depended on the gas pressure, on how hard the laser is excited, etc. Consequently, a long waveguide laser gain configuration maintains superior output power stability over a short gain length configuration because the large density of axial modes existing in a longer laser feedback cavity.
FIG. 1C illustrates the basic configuration of a slab laser discharge 10c. This technology was first introduced around the mid 1980""s and commercially around 1990. (Carbon Dioxide Slab Laser, by John Tulip, U.S. Pat. No. 4,719,639 issued January 1988, which is incorporated herein by reference). The larger width of the discharge region 10c provides a larger cross sectional area for the gain region and thus a larger volume for a given laser length which enables the design to yield higher output power per unit length of discharge than the configurations of FIG. 1A or 1B. As stated above, excellent waveguiding and diffusion cooling occurs in the vertical direction because of the narrow height (H) of the slab discharge region. Since single mode optical waveguiding does not occur in the horizontal direction of slab lasers because of the large width Fresnel number, Nfw, more complex optical resonators, such as unstable resonators, are utilized to discriminate against multimode oscillation occurring in the horizontal direction. The slab technology has enabled diffusion-cooled lasers to successfully compete with convectively cooled laser in excess of 500 W. Most slab lasers are operated pulsed rather than operated continuously in order to maintain uniform discharges across the wide area of the slab. In most cases separate gas ignition circuits are provided to ensure the discharge can be easily ignited when the discharge area of diffusion cooled lasers is large, such as for slab lasers.
FIG. 1D illustrates the basic feature of the inverted slab discharge laser configuration 10d. This is the newest diffusion cooled laser configuration and, as yet, is not commercially available nor has performance data been published (Rectangular Discharge Gas Laser, by Peter Chenausky, U.S. Pat. No. 5,748,663 issued May 5, 1999, which is incorporated herein by reference). The advantages for this configuration over the normal slab laser configuration are: 1) the ability to independently select and optimize the laser""s discharge pressure and excitation frequency, 2) having a higher discharge impedance in a lower capacitance structure for a better interfacing with solid state RF supplies, and 3) having an improved ability to supress arching within the discharge.
It is well accepted in the market place that diffusion cooled, slab CO2 lasers have size, cost, maintenance, and performance advantages over the convectively cooled CO2 laser up into the neighborhood of 500 W to 1000 W of output power. Approaching 1000 W and higher output powers, the convectively cooled CO2 lasers have the cost/performance advantages at this time. Additional attractions of diffusion cooled lasers are longer sealed-off operational and storage life times because of the ability to use superior vacuum tight technology and internal electrodes are not needed to excite the discharge, lower voltage requirements which are compatible with solid state RF power supply technology, higher reliability, lower maintainability cost because of no mechanical moving parts, and lower operational cost because there is no gas consumption with time. A sealed-off operation of over 20,000 hours without needing a gas refill is common place today. Waveguide diffusion cooled lasers have typically been limited to 150 to 200 W levels, well below the power capability of slab lasers.
The waveguide aperture dimensions used in waveguide gas lasers are much greater than the radiation wavelength emitted by the laser. Typical gas laser waveguides have intermediate aperture values equal to or greater than 100 times the wavelength of the radiation contained in the waveguides. (Chapter 3 entitled Radio Frequency Discharge Excited CO2 Lasers, by Denis R. Hall and Christopher A. Hill of the Handbook of Molecular Lasers, Edited by Peter K. Cheo, Marcel Dekker, Inc., 1987, which is incorporated herein by reference). Gas laser waveguides are of the special guides proposed in 1964 by E. A. J. Marcatili and R. A. Schmeltzer in Bell System Technical Journal, Vol. 43, page 1788, 1964, which is incorporated herein by reference. Such was directed toward long distance communication applications prior to the introduction of low loss glass fiber technology.
Some typical CO2 laser waveguide materials are alumna (Al2O3), aluminum (Al), Pyrex, oxide glass compounds, and beryllium (BeO). These materials are strongly absorbent at the IR wavelengths emitted by CO2 lasers (i.e. 9 xcexcm to 11 xcexcm). Aluminum is absorbent in the IR region of 9 xcexcm to 11 xcexcm because of the native oxide that always resides on its surface. Because of this absorption, only modes that have very small angle of reflection (i.e. glazing angles) off the absorbing walls of the guides have low loss. As a result, gas laser waveguides support no more than a few tens of practically relevant modes, each associated with small angle reflections at the guide wall. Consequently waveguides for gas lasers are very different than the glass waveguides utilized in fiber lasers.
In contrast to the focus of Marcatili and Schmeltzer (referenced above) on long distance optical propagation for telecommunications/data transmission, gas laser waveguide lengths are short and bending of the waveguides are not required nor desired. U.S. Pat. Nos. 4,787,090 and 5,610,936 and patent application Ser. No. PCT/US98/05055 revealed that folding of these waveguides present low loss that is very acceptable, especially with U.S. Pat. No. 4,787,090 and patent application Ser. No. PCT/US98/05055. Consequently, increasing the waveguide length has been a practical avenue for obtaining increased power output from waveguide lasers. Prior to U.S. Pat. Nos. 4,787,090 and 5,610,936 and patent application Ser. No. PCT/US98/05055, the length of gas laser waveguides were in the tens of centimeters. The longest present commercially available sealed waveguide CO2 laser utilizing the xe2x80x9cNVxe2x80x9d folded waveguide of FIG. 2B and patent application Ser. No. PCT/US98/05055 has a waveguide length of approximately 2.25 meters (see FIG. 2B) and has an output power exceeding 250 W. In experimental lasers using the Cantoni patent (U.S. Pat. No. 5,610,936), path lengths up to six meters have been demonstrated but only 200 W were obtained. This result indicates that the advantage of the tightly folded zigzag waveguide configuration associated with FIGS. 2A and 2B.
Until now, waveguide gas lasers having circular or square waveguide apertures, with either straight of folded configurations have been utilized. Slab lasers, which do not use waveguideing in the wide dimensions, are also known. Patent application Ser. No. PCT/US98/05055 was the first disclosure to indicate the advantage of CO2 laser waveguides having a rectangular configuration that still retained a simple optical resonator with all flat mirrors while also maintaining good mode quality (i.e. little or no side lobes adjacent to the main central lobe of the output beam). Patent application Ser. No. PCT/US98/05055 discloses a rectangular waveguide with a width to height aspect ration of 2 to 1 with the longer width waveguide dimension of the rectangular guide lying parallel to the width of the two opposite facing electrodes. The focus of patent application Ser. No. PCT/US98/05055 was on increasing the width of the guide so as to achieve an aspect waveguide ratio of width to height of approximately 2 to 1. Patent application Ser. No. PCT/US98/05055 also disclosed techniques for converting the elliptical output beam from the laser possessing such a rectangular guide into a circular beam having the desired diameter.
The above discussed shortcomings and other drawbacks and deficiencies of the prior art are overcome or alleviated by a laser of the present invention. In accordance with the present invention the laser comprises a housing defining a plurality of compartments therein, a waveguide disposed within the housing, the waveguide defining a plurality of rectangular waveguide channels having a substantially rectangular cross section for guiding a laser beam, a plurality of electrodes disposed in the plurality of compartments and positioned along opposite surfaces of the waveguide and at least one power supply connected to the plurality of electrodes. The channels having a prescribed width to height ratio for a prescribed total channel length for a given Fresnel number. At least one optical housing is provided. The optical housing is mounted to the laser housing, the optical housing including a plurality of beam turning mechanisms disposed within a plurality of compartments accessible for adjusting the beam turning mechanisms. The channels are disposed within the waveguide so as to subtend a prescribed angular orientation between adjacent channels. Distributed inductors are provided for suppressing the capacitance of the electrodes.